Delegations of Turkey, Russia, and Iran have decided to establish a Joint Working Group for meeting to set up a constitutional committee for Syria in Geneva, diplomatic sources said Wednesday.

The group will consist of experts from the three guarantor countries to facilitate the negotiations under the United Nations, the source said on condition of anonymity due to restriction on speaking to media.

On Tuesday, the Turkish Foreign Ministry said an agreement in principle, regarding the lists that were proposed by the Syrian regime and opposition, was discussed.

"During the meeting, they discussed the formation of a constitutional committee and its codes of practice which constitute an important step in the struggle of finding a political solution to the Syrian crisis," the ministry said.

UN Special Envoy to Syria Staffan de Mistura will meet on Friday with officials from Egypt, France, Germany, Jordan, Saudi Arabia, U.K., and the U.S.

On Sept. 18, he will address UN Security Council on the two-day talks.

The Geneva meetings held under the UN umbrella to end the seven-year-old civil war in Syria consists of four titles including transition, constitution, elections, and fight against terrorism.